1. Field of the Invention
This invention relates to a cigarette filter composed of a bundle of cellulose acetate filaments. More specifically, this invention relates to a cigarette filter composed of a bundle of cellulose acetate filaments having at least two different filament denier sizes, said filament bundle having a number average filament denier size of not more than about 4 denier, preferably not more than about 3.5 denier, the ratio of its weight average filament denier to its number average filament denier size being at least about 1.03, preferably at least about 1.05. The term "denier" as used herein is defined as the weight in grams of a filament 9000 meters long.
2. Description of the Prior Art
Present-day, cigarette filters are composed mainly of a bundle of cellulose acetate filaments and generally the filament bundle is an assembly of filaments having a single cross-sectional shape and denier. The recent tendency is toward filters having an increased tar filtration efficiency as a result of considerations of the health of smokers and changes in the taste of smokers. The increase of the tar filtration efficiency of filters can be achieved by using filaments having a fine denier size because a relatively large filament surface area can be obtained. However, when the denier size of filaments is decreased while keeping the total denier of the filament bundle unchanged, the pressure drop across the filament bundle increases with the increase in the filtration efficiency and a large amount of suction is required to smoke the cigarette. If the filling density is reduced so as to decrease the pressure drop, the filament bundle does not have sufficient hardness, and is not adaptable for manufacturing by a machine which molds the filter rod and connects it to the cigarette.
Some attempts have been made to overcome these difficulties and to obtain filters having a high filtration efficiency and a low pressure drop. For example, it has been suggested to retain hardness and filtration efficiency and reduce pressure drop by molding a filament bundle composed of filaments having a fine denier (for example, Japanese patent applications (OPI) Nos. 7799/72 and 7500/74 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application.")). However, a special device is required to produce the molded filters and continuous production of such filters is difficult. Moreover, it is difficult to perform production economically.
It has also been proposed to reduce pressure drop by providing vents in the filter wrapping paper, thus permitting air flow (for example, Japanese patent applications (OPI) Nos. 79099/77 and 114100/77). However, the balance of the components making up the tobacco smoke changes which tends to impair the flavor and smoking taste of the cigarettes.
Cigarette filters composed of a group of fibers having different filament denier sizes have also been suggested. For example, a filter has been suggested in which fibers having a large filament denier size are present near the central axis of the filter and fibers having a fine filament denier size are present in the peripheral portion of the filter and the fibers are aligned concentrically (for example, Japanese patent application (OPI) No. 79098/77). A filter having a high filtration efficiency and a good smoking taste is supposed to be obtained by allowing smoke having a low filtration efficiency to pass through the central part of the filter and smoke having a high filtration efficiency to pass through the peripheral portion. However, the manufacturing process for this filter is complex, and the filter is difficult to produce by an ordinary filter making machine. Thus, a special manufacturing apparatus is required. Furthermore, because fibers of a fine denier size are aligned in the peripheral portion of the filter, its hardness tends to be insufficient when the degree of filling is such as to give a suitable pressure drop. It is difficult to produce such a filter industrially with good efficiency.
It has also been suggested to provide a filter of non-woven cloth by aligning fibers of different filament denier sizes randomly and molding them into a web (Japanese patent application (OPI) No. 133499/76). This technique is quite different from the process for manufacturing a filter composed of aligned filament bundles, and it is difficult to utilize this technique based on presently established techniques for producing beautiful filters at high speeds.